1. Field of the Invention
The invention is a method of and circuit for controlling the evolution time interval of a laser output pulse and more particularly to such a method and circuit for producing narrow spectral bandwidths for use in making precise spectral measurements.
2. Description of the Prior Art
A recurring problem confronting lasers used for precise spectral measurements is the need for a narrow linewidth laser. A known approach to this problem is the use of a self-injection locking lasers in which a laser is operated in a high loss condition during the initial portion of the pulse evolution time interval. By operating in a high loss condition, the resulting laser output pulse will have made many round trips through a laser resonator. On each round trip, line narrowing elements operate on the radiation in the laser pulse to successively narrow its spectral linewidth. Thus, by increasing the loss, and thus the number of round trips, the linewidth of the laser becomes narrower.
Once a small laser pulse has evolved in a high loss resonator, the loss is switched in a single step or level action to a low loss state. In the low loss state, the majority of the energy is extracted in an efficient manner while retaining the narrow spectral linewidth of the initial pulse. Typically, a Q-switch, either an electro-optic or an acoustic-optic model, is used to effect switching to a low loss condition.
While this method of single level switching is operable, the time at which the laser is switched directly from a high loss condition to a low loss condition is a critical limitation. If the single level of switching occurs too early, insufficient round trips will have been completed leaving a wider than desired linewidth. Thus the output pulse evolution time interval is too short.
In the known method, this effect is counteracted through the use of an optical sensor to detect the level of the output laser radiation from an output element such as an output mirror. When the output laser radiation builds to a sufficient level, such an optical sensor detects this level and switches the laser to a low loss condition in a single level switching action. Although this is an improvement, it does not directly regulate the number of round trips required to achieve the desired spectral linewidth.
Thus, if the pumping level is too high, a laser output pulse will evolve in the resonator quickly, thus counteracting the effect of the line narrowing elements. If the pumping level is too low, the laser may never come to threshold and lasing preventing altogether. In addition, this method introduces an indeterminacy or jitter into the time at which the laser output pulse occurs.
As the output pulse evolution time interval depends on the level of the pump, the time at which the pulse occurs may be set by the pumping level rather than an external clock. While external clocks are known to be highly accurate, the level of pumping is usually not accurately known or controllable. Thus, though any resulting jitter may not be much, probably on the order of a microsecond or so, it may cause problems if precise ranging is to be performed.